System and method for mitigating fading of a signal at a radio receiver

ABSTRACT

The present invention provides a method and system for mitigating fading and/or poor reception at a receiver. The receiver includes configurations each of which can provide different reception characteristics. The receiver evaluates the reception quality of the radio signal with the antenna in a first configuration and with the antenna in at least a second configuration and selects the antenna configuration that has the best evaluated reception for use until a subsequent iteration, when the process is repeated. The antenna configurations can correspond to configurations wherein reception is favored in different directions or to configurations wherein different antennas are selected, each antenna being spaced from each other antenna. The method can also improve the reception of a signal transmitted by selecting an antenna configuration for transmissions which provides improved reception quality at the destination receiver.

This application is a 371 of PCT/CA02/01637, filed Oct. 31, 2002(designating the U.S.; and which published in English in WO 03/039032 onMay 8, 2003), which claims the benefit of Canadian Patent ApplicationNo. 2,361,186, Nov. 2, 2001, incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a system and method for mitigatingsignal fading and/or poor reception experienced at a radio receiver.More specifically, the present invention relates to a system and methodto mitigate signal fading and/or poor reception at a receiver byselecting, according to a reception quality, a direction and/or locationat which to receive and/or transmit the signal.

BACKGROUND OF THE INVENTION

It is well known that radio signals can suffer from a variety ofinfluences that result in a radio receiver experiencing fading of thereceived radio signal. For example, a radio signal received at areceiver can be subject to Rayleigh fading, other multi-path, orenvironmental fading influences.

Rayleigh fading, and other multi-path fading effects, result whenmultipath versions of a signal destructively interfere to attenuate thesignal arriving at a receiver. The fading experienced at a receivertypically is unique to that receiver, as it depends upon the particularmultiple paths the signals travel between the transmitter and thereceiver. Such fading is a significant problem in communications system,especially mobile systems where movement of the receiver and changes inthe environment both contribute to changing multipath signals.

Prior attempts to deal with Rayleigh fading and the like haveconcentrated on providing redundant information in the signal via errorcorrection coding, symbol repetition and interleaving of transmittedinformation, to allow a receiver to reconstruct symbols lost during afade experienced at the receiver. While such techniques allow systemdesigners to provide a selected probability of reception of a signal,this is achieved at a cost of reduced system throughput, as redundantinformation must be transmitted, reducing the bandwidth utilizationefficiency of the transmission.

Various other approaches to dealing with Rayleigh fading and the likehave also been proposed. For example, the third generation partnershipproject (“3GPP”), which is working to develop a next generation wirelesscommunication system, has proposed a “compressed mode” of operation.Specifically, and as described in more detail in the documents availablefrom the web site (www.3gpp.org) of the 3GPP organization, the 3GPP airinterface has been defined as a slotted-frame architecture wherein data(voice data and “pure” data such as HTTP or FTP data) is typicallytransmitted from the base station to receivers in frames of tenmillisecond duration and each frame comprises fifteen time slots ofdata. A receiver experiencing poor reception from a base station, due toa fade, or other factors, can inform the base station that it wishes toenter compressed mode for one or more of the next frames sent to it.Once the transmitter at the base station agrees, the agreed compressedframes are transmitted to the receiver.

Each compressed frame contains some agreed number of empty (i.e.—they donot contain data) slots at the end of the frame. The receiver, knowingthat the agreed number of slots will not contain data intended for it,is free during the transmission time of those empty slots to attempt toreceive signals from other transmitters, such as the pilot signalbroadcast by each other base station, to evaluate the receiver's abilityto receive those other transmitters. In other words, transmission ofdata to the receiver occurs only in a portion of the frame and nothingis transmitted in the balance of the frame when the receiver is notlistening to that transmitter.

If the receiver can better receive another transmitter than thetransmitter it has been listening to, the receiver can inform thecurrent transmitter that it requires a hand off to the better receivedtransmitter. In this manner, which can be considered a form oftransmitter diversity, a receiver experiencing a fade or other poorreception characteristics from one transmitter can switch to anothertransmitter which it can receive at a better condition.

One problem with the 3GPP compressed mode is that radio transmissioncapacity is wasted whenever the empty slots are transmitted. It ispresently contemplated by 3GPP that typically seven of the fifteen slotsin a frame will be empty during compressed mode. Another problem with3GPP compressed mode is that latency is increased during compressed modeas data which would otherwise be transmitted in the empty slots isdelayed until a subsequent frame. Yet another problem with the 3GPPcompressed mode is that it requires coordination between the transmitterand the receiver and this adds delay and overhead servicing requirementsto the communication system. Thus, there will always be some additionaldelay in how quickly a receiver can evaluate its reception of othertransmitters when it is experiencing a fade, or other poor receptioncharacteristics, from the transmitter it is presently receiving. Also,compressed mode is only employed once poor reception is beingexperienced and it is not used proactively, to prevent unacceptablereception from occurring, where possible. Finally, 3GPP compressed modepresumes that another transmitter is available in the system and that ahand off of responsibility for the receiver can be achieved between thetransmitters.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel system andmethod to mitigate fading and/or other poor reception characteristics ofa radio signal experienced at a receiver which obviates or mitigates atleast some of the above-identified disadvantages of the prior art.

According to a first aspect of the present invention, there is provideda method of receiving a radio signal at a receiver, comprising the stepsof:

(i) receiving a first portion of the radio signal at the radio receiverthrough a first selected antenna configuration of an antenna diversitymechanism and evaluating the reception quality of the portion;

(ii) selecting another antenna configuration as a second antennaconfiguration and receiving a second portion of the radio signal at theradio receiver through the second antenna configuration and evaluatingthe reception quality of the portion, the second portion being smallerin transmission duration than the first portion;

(iii) repeating steps (i), (ii) and (iii) with the first antennaconfiguration being the antenna configuration which provided the bestreception quality in the previous iteration of the method.

Preferably, there are at least three antenna configurations. Alsopreferably, the reception quality for each antenna configuration isdetermined at the start of the method and stored in the receiver. Eachtime an evaluation of the reception quality of an antenna configurationis determined, the stored value is updated and the second selectedconfiguration in step (ii) is the configuration with the next beststored value.

Also preferably, there are at least three antenna configurations andstep (ii) is repeated to receive a portion of the signal for eachconfiguration not used to receive the first portion and theconfiguration with the evaluated best reception quality is employed onthe next iteration to receive the first portion of the signal. Alsopreferably, the signal is transmitted as a slotted frame and the firstportion comprises multiple slots.

Also preferably, the antenna configurations comprise configurations ofthe antenna diversity mechanism whereby each configuration correspondsto a different favored reception direction. As a preferred alternative,the antenna configurations comprise different antennas, each antennaspaced from each other antenna.

According to another aspect of the present invention, there is provideda method of receiving a radio signal at a receiver, comprising the stepsof:

(i) receiving a first portion of the radio signal at the radio receiverwith an antenna configured to favor a first selected direction andevaluating the reception quality of the first portion;

(ii) receiving a second portion of the radio signal selecting with theantenna configured to favor a second selected direction and evaluatingthe reception quality of the second portion, the second portion beingsmaller in transmission duration than the first portion;

(iii) setting the first selected direction to be the favored directionwhich provided the best reception quality in the previous iteration ofthe method and repeating steps (i), (ii) and (iii).

Preferably, the radio signal is divided into at least first, second andthird portions, the second and third portions having a transmissionduration of less than the first portion and where step (ii) is alsoperformed with the antenna configured to favor a third direction for thethird portion of the radio signal.

According to another aspect of the present invention, there is provideda receiver for receiving a radio signal divided into at least a firstportion and a second portion, the second portion having a shortertransmission duration than the first portion, the receiver comprising:

an antenna diversity mechanism having at least two antennaconfigurations;

a controller to select one of the at least two antenna configurations toreceive the first portion of the radio signal and to select a second ofthe at least two antenna configurations to receive the second portion ofthe radio signal; and

a reception quality evaluator to determine the reception quality atwhich the portions of the radio signal are received, the controllerbeing responsive to the determined reception qualities to select theantenna configuration with the best determined reception quality to bethe selected antenna configuration for reception of the first portion ofa subsequent radio signal.

Preferably, the antenna diversity mechanism comprises a steerableantenna and each antenna configuration corresponds to the favoring of adirection from which to receive the radio signal. Also preferably, theantenna diversity mechanism has four antenna configurations, eachconfiguration corresponding to a directional quadrant of about ninetydegrees from which reception of the radio signal is favored.

According to yet another aspect of the present invention there isprovided a wireless telecommunication system, comprising:

a wireless base station for transmitting a radio signal; and

a subscriber station for receiving the radio signal divided into atleast a first portion and a second portion, the second portion having ashorter transmission duration than the first portion, the subscriberstation comprising:

-   -   an antenna diversity mechanism having at least two antenna        configurations;    -   a controller to select one of the at least two antenna        configurations to receive the first portion of the radio signal        and to select a second of the at least two antenna        configurations to receive the second portion of the radio        signal; and    -   a reception quality evaluator to determine the reception quality        at which the portions of the radio signal are received, the        controller being responsive to the determined reception        qualities to select the antenna configuration with the best        determined reception quality to be the selected antenna        configuration for reception of the first portion of a subsequent        radio signal.

Preferably, the subscriber station is also operable to transmit a radiosignal to the base station, the controller further selecting one of theat least two antenna configurations to transmit the radio signal to thebase station.

The present invention provides a method and system for mitigating fadingand/or poor reception at a receiver. The receiver includes an antennadiversity mechanism which can be operated in a variety of antennaconfigurations each of which can provide different receptioncharacteristics. The receiver evaluates the reception quality of theradio signal with the antenna in a first configuration and with theantenna in at least a second configuration and selects the antennaconfiguration that has the best evaluated reception for use until asubsequent iteration when the process is repeated. The antennaconfigurations can correspond to configurations wherein reception isfavored in different directions or to configurations wherein differentantennas are selected, each antenna being spaced from each otherantenna.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the attached Figures, wherein:

FIG. 1 shows a schematic representation of a pair of transceivers inaccordance with an embodiment of the present invention;

FIG. 2 shows a schematic representation of an antenna diversitymechanism employed in an embodiment of the present invention;

FIG. 3 shows a schematic view of a subscriber station in accordance withan embodiment of the present invention;

FIGS. 4 a, 4 b, 4 c, 4 d and 4 e show frame structures of signals;

FIG. 5 shows a flowchart of an embodiment of the process of switchingthe antenna diversity mechanism of FIG. 2 to improve reception quality,in accordance with the present invention;

FIG. 6 shows a flowchart of an embodiment of the process of switchingthe antenna diversity mechanism of FIG. 2 to improve transmissionquality, in accordance with the present invention; and

FIG. 7 shows a transceiver having another antenna diversity mechanism inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a communications network 10, such as a wireless local loopsystem, including a first transceiver, such as base station 20 and asecond transceiver, such as subscriber station 24, which arecommunicating by a radio signal 28 in accordance with an embodiment ofthe present invention. As is known, radio signal 28 typically travelsover many signal paths 28 (for clarity only two signal paths 28 a and 28b are shown in the Figure) before being received at an antenna diversitymechanism of subscriber station 24. As shown in the Figure, signal path28 b is a greater distance, the signal having been reflected off of anobject 36 such as a building, than signal path 28 a which is a directpath. If the present invention was not employed, a fade could beexperienced at subscriber station 24 when the signals arriving overpaths 28 a and 28 b, etc., tend to cancel each other at subscriberstation 24 through destructive interference.

FIG. 2 shows an embodiment of antenna diversity mechanism 32 ofsubscriber station 24 that uses a steerable antenna 34 in more detail.An example of a suitable steerable antenna 34 can, for example, besimilar to that described in U.S. patent application Ser. No.09/775,510, assigned to the assignee of the present invention and thecontents of which are incorporated herein by reference. Another exampleof a suitable steerable antenna can be similar to that described in U.S.patent application Ser. No. 09/899,927, assigned to the assignee of thepresent invention and the contents of which are incorporated herein byreference. Both of these antennas allow for the selection betweenantenna configurations wherein favored direction for reception and/ortransmission can be selected, the favored direction being the directionwhich the antenna is most sensitive in and/or directs the mosttransmitted power in. Other examples of suitable antenna diversitymechanisms 32, such as the use of multiple antennas, will occur to thoseof skill in the art.

As shown in the example of FIG. 2, steerable antenna 34 includes anantenna element 40 and four steering elements 44 a, 44 b, 44 c and 44 dwhich can be switched between floating or electrically grounded statesto “steer” antenna element 40 to favor reception, and transmission, ofradio signals in the general direction of one of four roughly ninetydegree sectors 48 i, 48 j, 48 k or 48 m and thus steerable antenna 34has four antenna configurations. As will be apparent to those of skillin the art, steering elements 44 can be switched individually, or inpairs, etc. to yield different characteristics of favored reception andtransmission directions. It is also contemplated that greater or fewerthan four steering elements 44 can also be employed to achieve favoredreception or transmission over greater or lesser directions than roughlyninety degree sectors, if desired. For example, antennas having moresteering elements 44 can be employed to provide antenna configurationsfavoring sectors of six sectors of sixty degrees or eight sectors offorty-five degrees, etc. For reasons of clarity only, the discussionherein refers only to antenna configurations of sectors havingtransmission and/or reception angle of about ninety degrees, howeverconfigurations of sectors of greater angles or lesser angles areintended to be included within the scope of the present invention.

Subscriber station 24 is shown in more detail in FIG. 3. As mentionedabove, subscriber station 24 includes an antenna diversity mechanism 32which comprises an electrically steerable antenna 34 and a controller 50to select between the available antenna configurations. Subscriberstation 24 also includes a radio 54 and a modem 58 which, in turn,receive signals over radio link 28 and convert the received signals intodata. A processor 62, such as an Intel StrongArm processor, processesthe received data and provides it to data devices 66 and/or telephonydevices 70 connected to subscriber station 24. Radio 54 includes areception quality evaluation function which determines an appropriatereception metric, such as a signal to noise ratio, and provides thatmeasurement to processor 62. Processor 62 is operable to respond to thismeasurement to instruct controller 50 to select an antenna configurationfor steerable antenna 34, as described below.

In a present embodiment of the invention, data is transmitted from basestation 20 to subscriber stations 24 in the slotted frame 100 structure,shown in FIG. 4 a, which is similar to that proposed by the 3GPP body.As with the 3GPP structure mentioned above, each frame 100 includesfifteen slots 104. In this embodiment, CDMA is employed as a multipleaccess technique and a chip rate of three-million eight-hundred andforty thousand chips per second is employed. Thus, for frames 100 of tenmilliseconds duration, a frame will comprise thirty-eight thousand, fourhundred chips, with each slot 104 in frame 100 comprising two-thousand,five-hundred and sixty chips. However, as will be apparent to those ofskill in the art, the present invention is not limited to use with sucha slotted frame structure and the present invention is compatible withmany other transmission structures.

FIG. 4 b shows a frame 100 constructed in accordance with the prior artcompressed mode proposed by the 3GPP organization, wherein the hatchedslots are empty, i.e. —contain no data intended for a receiver, when thetransmitter and receiver have agreed to enter compressed mode. Duringtransmission of the empty slots, the receiver can attempt to locateanother transmitter which it can receive at a better quality, typicallyby evaluating the reception quality of the pilot signal transmitted bysuch other transmitters.

In contrast, in the present invention, subscriber station 24 selects aninitial sector 48 from which it can best receive signals from basestation 20. This selection will first be performed as part of the normalacquisition process whereby subscriber station 24 determines the basestation 20 that it will be serviced from. Essentially, as part of theacquisition process, the reception quality of signals from base station20 can be determined for each sector 48 and the sector 48 with the bestreception quality will be initially selected. Other techniques forselecting an initial sector, including a random selection, can also beemployed, as will be apparent to those of skill in the art.

Then, from time to time, subscriber station 24 will determine if it canbetter receive signals from base station 20 from an alternative sector48. For example, if subscriber station 24 is receiving a signal frombase station 20 from sector 48 j at a given signal to noise ration (SNR)or any other appropriate metric of reception quality, subscriber station24 can at an appropriate time switch to another sector, such as 48 k, todetermine if it can receive the signal at a better SNR from thatdirection. If, in this example, sector 48 k provides a better SNR thanthat presently experienced from sector 48 j, subscriber station 24 willcontinue to receive from sector 48 k. If sector 48 k does not provide abetter SNR than sector 48 j, subscriber station 24 can either return toreceiving from sector 48 j or can evaluate reception from other sectors(48 i and 48 m), as discussed in more detail below.

Most CDMA systems employ a RAKE receiver whose fingers are aligned withthe strongest multipath received signals to allow the multipath signalsto be combined to achieve a higher SNR than would otherwise be possible.Each finger of a RAKE receiver has a set of parameters determined for itwhich define the reception characteristics of the multipath version ofthe signal it is receiving. With the present invention, the values forthese parameters are, determined for each sector 48 at start up ofsubscriber station 24. The parameters for a sector 48 are then updatedwhenever signals are received from that sector 48 and the latest valuesfor these finger parameters in that sector 48 are stored wheneversteerable antenna 34 is switched to another sector 48. If a RAKEreceiver is not employed in subscriber station 24, for example if anequalizer is employed instead, the relevant reception parameters for theequalizer for each sector 48 are determined and stored. Thus, subscriberstation 24 maintains the last set of reception parameters determined forreceiving signals from each of sectors 48 i, 48 j, 48 k and 48 m.

With the stored parameters, switching of steerable antenna 34 to anothersector 48 can be achieved very quickly, for example in a single chipinterval. Specifically, the appropriate stored reception parameters canbe loaded into the RAKE receiver or equalizer while elements 44 aregrounded or floated to switch sectors 48 to receive the signal from thenewly selected sector 48. Thus, very rapid evaluations of receptionquality from a sector 48 can be performed with the present invention andit is contemplated that evaluations of reception quality in sectors 48can be performed as needed in slot or even sub-slot intervals.

As rapid (in some cases slot or sub-slot) evaluations of receptionquality can be achieved, data can be transmitted by base station 20 inevery slot 104 of frame 100 in the present invention, unlike the 3GPPcompressed mode. At worst, should subscriber station 24 be receiving aframe 100 and select a different sector 48 with too low a receptionquality to accurately receive the contents of a slot 104, the contentsof that slot 104 can be lost. However, the error correcting coding andsymbol repetition employed for the data transmitted through frame 100will typically allow recovery and/or reconstruction of a missed slot 104or other portion of a frame 100. If insufficient error correction and/orrepetition is present, subscriber station 24 can request retransmissionof the contents of the missing slot 104 or other portion of frame 100.If sub-slot evaluations are employed by subscriber station 24, an evenbetter likelihood of recovery and/or reconstruction of a slot 104 can beachieved.

Several different methods of operation are contemplated in the presentinvention. A first method is to perform a switch to a different sector48 when receiving the last slot 104 of each frame 100, or of each Xthframe 100, where X can be one to ten, or more. For example, as shown inFIG. 4 c, subscriber station 24 can be receiving from sector 48 i forthe first fourteen slots 104 i of frame 100 and a switch of steerableantenna 34 is then performed to receive from another selected sector,such as 48 j, for the fifteenth slot 104 j. If the signal from basestation 20 is received at subscriber station 24 with at least as good areception quality from sector 48 j as from sector 48 i, receptioncontinues for the next fourteen subsequent slots 104 j (from the nextframe 100 not shown) from sector 48 j. If the reception of slot 104 jfrom sector 48 j is worse than the reception quality of slots 104 i fromsector 48 i, reception returns, for the next fourteen subsequent slots104 i (from the next frame 100) to sector 48 i.

In this embodiment, a table of the last experienced reception qualities(LERQ) from each sector 48 is maintained in subscriber station 24 and isupdated with this latest information as are the cached receptionparameters. The selection of which sector 48 to test at the nextinterval can be made according to the next highest reception qualitylevel indicated in the LERQ table.

As an example, given the LERQ Table 1 of Appendix A, and with subscriberstation 24 receiving via sector 48 i, sector 48 j would be tested next.If sector 48 j is, when tested, received at a lower reception qualitythan 48 i, subscriber station 24 switches back to sector 48 i andupdates the LERQ table to the value for 48 j in Table 2 of Appendix A.Sector 48 m now has the highest historical reception quality of thethree sectors other than 48 i and will thus be the sector next tested bysubscriber station 24. If, when tested, subscriber station 24 has abetter reception via sector 48 m, it continues to receive via thatsector and updates the LERQ table to that shown in Table 3 of AppendixA. On the next switch, subscriber station 24 will again consider sector48 i as it has the highest historical reception quality. If sector 48 mhas a better reception quality than sector 48 i which could, forexample, be experiencing a fade, reception returns to sector 48 m andthe LERQ table is updated as shown in Table 4 of Appendix A. In thiscase, at the next switch, sector 48 k will be tested.

As will be apparent to those of skill in the art, other selectionmethods than the LERQ table can be employed, in which case the LERQtable can be omitted. For example, it may be that it is commonly thecase that if a sector 48 suffers a fade, that the most likely sectorwith better reception will be one of the sectors that are at ninetydegrees to the affected sector. For example, sectors 48 i and 48 k mayhave the best reception at one time, with sector 48 i being the selectedsector, but as the reception quality experienced at sector 48 idecreases, so does the reception quality which would be experienced atsector 48 k. Thus, sectors 48 m and 48 j should be evaluated and/oremployed and one of them will be selected and evaluated.

It may also be the case that no correlation exists between experiencedreception qualities prior to a fade and those during a fade.Accordingly, a new sector 48 to be evaluated can be selected randomlyand this process can repeat until each sector 48 has been evaluated andthe sector 48 yielding the best result is selected.

In any event, the process of switching sectors for reception isillustrated in the flowchart of FIG. 5 wherein at step 200, a predefinedportion of a transmitted signal, such as fourteen of fifteen slots 104of a frame 100, is received and the reception quality determined. Asused herein, the tern portion is intended to comprise a time duration ofthe transmitted signal and can be defined as a time period or as anumber of transmitted traffic bits, chips or any other suitable quantumof the transmitted signal. At step 204, an alternative antenna directionis selected, based upon values in the LERQ table or based upon any othersuitable criteria, including performing a random selection. At step 208,the antenna is switched to the selected direction and the receptionquality is evaluated and determined for a selected portion of thesignal, for example one slot. If a LERQ table or other record ofreceived transmission qualities is being employed, this record isupdated with the determined reception value.

As-will be apparent to those of skill in the art, at the start up ofsubscriber station 24, reception is performed in each direction for atime period, such as five frames, which is selected to ensure a goodevaluation of reception and/or transmission characteristics and the RAKEreceiver, equalizer or other receiver parameters. After this initialcharacterization/determination is performed, the process of FIG. 5commences.

At step 216 a determination is made as to whether the direction selectedin step 204 provides a better reception quality than the directionemployed in step 200. If an improvement has been obtained, the processreturns to step 200 and employs the direction selected in step 204. Ifno improvement was obtained, the process proceeds to switch back to theprevious antenna direction at step 220 and then returns to step 204 andthe process repeats.

Another technique for managing the present invention is to examine eachof the three alternative sectors 48 during reception of a frame 100. Inthis embodiment, the first twelve slots 104 in each frame 100 can bereceived via one sector 48 i and each of the three other sectors 48 j,48 k and 48 m will then be tested for a respective one of the threeremaining slots 104 j, 104 k, 104 m as shown in FIG. 4 d. The sector 48with the best determined reception quality is selected for reception ofthe first twelve slots 104 of the next frame 100.

Yet another technique for managing the present invention comprisesperforming sub-slot testing of the three alternative sectors 48. Thus,the first fourteen slots 104 i of a frame 100 are received via onesector 48 i and each of the three alternative sectors 48 j, 48 k and 48m is tested for about one-third each of the fifteenth slot 104 jkm offrame 100 as shown in FIG. 4 e. Again, the sector 48 with the bestreception quality is selected for receiving the first fourteen slots ofthe next frame 100.

If fading occurs quickly, relative to the duration of frames 100, suchas when the receiver or transmitter or both are mobile, sub-slot testingcan be employed more than once per frame. For example, four slots 104can be received from one sector 48 then the other three sectors 48 canbe examined for a fifth slot 104 with the sector 48 with the bestreception being selected for the next four slots 104 and the processrepeated. Thus, the best sector is determined from all sectors 48 threetimes per frame 100.

As will be apparent, each of the above mentioned operating techniqueshas a different characteristic in responding to fades or other radioreception factors. For example, relatively slow changes, such as typicalRaleigh fading in a fixed wireless circumstance, can be adequatelymanaged by the first mentioned technique while fading in a mobilitycircumstance may be better dealt with one of the sub-slot samplingtechniques.

It is contemplated that the rate at which other directions are evaluatedcan be varied during operation, depending upon the operating conditionsexperienced in the network For example, when a subscriber station 24 isemployed in a fixed circumstance, i.e. —the subscriber station 24 is notbeing moved in use, a relatively low rate of evaluations can beemployed, such as once every two frames 100, etc. If that subscriberstation 24 subsequently is used in a mobile circumstance, i.e. —thesubscriber station 24 is used while moving between locations at arelatively rapid rate, system 20 can switch to evaluating directions toand from that subscriber station 24 more frequently, such as once perframe or even more frequently. Thus, some subscriber stations 24 can beevaluating their transmission and/or reception directions at a slow ratewhile other subscriber stations 24 is system 20 employ a faster or muchfaster rate of evaluation.

As mentioned above, it is also contemplated that antenna diversitymechanisms with greater or fewer than four configurations can beemployed. For example, six sixty-degree sectors can be employed and theabove-mentioned processes can be performed to select the one sector ofthe six with the best reception quality.

In the system and method described above, a simplifying assumption hasbeen made in that transmissions from subscriber station 24 to basestation 20 will be transmitted in the same sector 48 from which the bestreception of signals from base station 20 has been achieved. As will beapparent to those of skill in the art, this assumption is notnecessarily correct as the downlink and uplink transmission paths candiffer significantly, at least in frequency division duplexed (FDD)systems. Accordingly, the present invention can also switch sectors 48for transmitting from subscriber station 24 to a base station 24.

Specifically, if system 10 is a time division duplexed (FDD) system(i.e. —a subscriber station 24 can only either transmit or receive asignal at a given time, over the same frequency) then steerable antenna34 can be switched to appropriate sectors 48 when receiving, asdescribed above. When transmitting, subscriber station 24 employs thesame sector 48 last selected for reception, as the path traveled by thereceived signal will be the same path traveled by the signaltransmitted.

If system 10 is a frequency division duplexed (FDD) system (i.e. —asubscriber station 24 can both receive and transmit signals at the sametime, over different frequencies), then the antenna diversity mechanismcan comprise two antennas 32 in subscriber station 24, one fortransmitting and one for receiving. One of these antennas 32 willevaluate and be switched to the sector 48 with the best receptionquality and the other will evaluate and be switched to the sector 48with the best transmission quality. In the latter case, whentransmitting subscriber station 24 employs information sent in the lasttransmission from base station 20 to subscriber station 24 to select asector 48 on which to transmit. Specifically, base station 20 willinform subscriber station 24 of the quality at which its lasttransmission was received at base station 20 (this information can beexplicitly provided to subscriber station 24 or can be implicitlyderived by subscriber station 24 from power control information or otherinformation sent to the subscriber station 24 from base station 20).

Subscriber station 24 maintains a record of the last transmissionreception quality (LTRQ) experienced at base station 20 for each of itssectors 48, as reported by base station 20, and will switch steerableantenna 34 to the sector 48 with the best recorded LTRQ beforetransmitting. If the base station 20 informs the subscriber station 24that its transmission reception quality is decreasing, subscriberstation 24 will update the recorded quality for that sector 48 in itsLTRQ record and will determine which sector 48 now has the best recordedquality and that sector 48 is then selected for the next transmission.Alternatively, subscriber station 24 can transmit from each sector 48for a portion of a frame 100 and base station 20 will advise the qualitywith which each corresponding portion was received. Subscriber station24 will then select the sector which base station 20 received at thebest quality for transmitting the bulk of the next frame 100. Thus,subscriber station 24 may receive transmissions from base station 20from sector 48 i and yet transmit to base station 20 through sector 48k, etc.

In the FDD case, a process for switching sectors 48 for transmission isillustrated in the flowchart of FIG. 6 wherein at step 250 atransmission from subscriber station 24 to base station 20 occursthrough a sector 48. As discussed above, this transmission can comprisea portion of a frame, an entire frame or even multiple frames. The basestation 20 receives the transmission and determines the transmissionreception quality and at step 254, informs subscriber station 24 of thetransmission reception quality it experienced and subscriber station 24makes a record of the transmission reception quality. At step 258 adetermination is made as to whether the previous transmission receptionquality from any other sector is better than the transmission receptionquality of the sector presently employed. If the transmission receptionquality from the present sector is at least as good as the transmissionreception qualities last recorded for the other sectors, the presentsector will be used for the next transmission and the process returns tostep 250. If the transmission reception quality is not as good as thetransmission reception quality last recorded for any other sector, atstep 262 the sector with the best last recorded transmission receptionquality is selected for the next transmission and the process returns tostep 250.

By proactively switching between sectors 48 wherein an improvedreception quality is obtained at the subscriber station 24, transmissionpower allocated by base station 20 to the transmission to subscriberstation 24 can likely be reduced, allowing this power to be assigned toother communications from base station 20. By proactively switchingbetween sectors 48 wherein improved transmission reception is obtainedat base station 20, more efficient transmission techniques, such asusing less FEC coding or symbol repetition, can be employed to increasethroughput from subscriber station 24 to base station 20. Similarly, byswitching between sectors 48 to improve reception at base station 20,either less power can be employed for the transmission or more data canbe transmitted (higher data rate) for the same power, in both casesimproving overall transmission capacity to base station 20 fromsubscriber stations 24 in system 20.

It is also contemplated that in certain circumstances, a subscriberstation 24 will receive signals from a first base station 20 andtransmit signals 24 to a second base station 20 (not shown). In such acase, the sector 48 to receive from will be determined relative to firstbase station 20 and the sector 48 to transmit from will be determinedrelative to second base station 20 and second base station 20 willinform first base station 20, for example through a land line backhaulnetwork (not shown), as to the reception quality second base station 20experienced and first base station 20 will forward this information tothe subscriber station 24.

FIG. 7 shows another embodiment of a subscriber station 300 inaccordance with the present invention. In this embodiment, instead ofhaving an antenna diversity mechanism 32 comprising a steerable antenna34, the antenna diversity mechanism 34 comprises multiple spacedantennas 304 at subscriber station 300, each antenna 304 located atleast ½ a wavelength of the radio signal from each other antenna 304.Subscriber station 24 includes a suitable controller to select aconfiguration comprising one of the multiple antennas 304 fortransmission and/or reception. In another alternative, each antenna is adirectional antenna and is or can be aimed in a different direction thaneach other antenna 304 in addition to being spaced from them as with thefirst alternative.

As with the steerable embodiments described above, in the embodiment ofFIG. 7 one of antennas 304 is selected for reception and/ortransmission. Specifically, the antenna 304 which has the best receptionquality, compared to the other antennas 304, is determined atappropriate intervals and that antenna is then selected for receptionuntil the next interval. In FDD embodiments, the antenna 304 which hasthe best transmission quality is also determined at appropriateintervals and that antenna 304 is then selected for transmission untilthe next interval.

The above-described embodiments of the invention are intended to beexamples of the present invention and alterations and modifications maybe effected thereto, by those of skill in the art, without departingfrom the scope of the invention which is defined solely by the claimsappended hereto.

TABLE 1 Sector Reception Quality 48i 57 48j 48 48k 39 48m 41

TABLE 2 Sector Reception Quality 48i 57 48j 37 48k 39 48m 41

TABLE 3 Sector Reception Quality5 48i 57 48j 37 48k 39 48m 59

TABLE 4 Sector Reception Quality 48i 22 48j 37 48k 39 48m 59

1. A system for transmitting and receiving a radio signal with a slottedframe structure, said system comprising: a first transceiver operable totransmit said radio signal; a second transceiver operable to receivesaid radio signal from an initial antenna configuration selected from aplurality of possible antenna configurations via antenna diversity;where said second transceiver can make a determination of receptionquality for said radio signal from at least one alternative antennaconfiguration selected from said plurality of possible antennaconfigurations during the reception of a slot containing data from saidfirst transceiver in said initial antenna configuration; where saidsecond transceiver can make a selection of an antenna configuration toreceive said radio signal from between said initial antennaconfiguration and said at least one alternative antenna configuration;where said selection of said alternative antenna configuration is madebased upon a history of reception qualities achieved from each of saidplurality of possible antenna configurations that have occurred at saidsecond transceiver; where said selection of said alternative antennaconfiguration is made using a combination of random selection andhistorical selection; and where of one of said random selection and saidhistorical selection is chosen based upon the rate of fading experiencedat said second transceiver.
 2. The system of claim 1, where saiddetermination of reception quality is made after a regular time period.3. The system of claim 2, where said regular time period occurs at leastonce over the duration of a slot.
 4. The system of claim 1, where saiddetermination of reception quality is made whenever reception qualitydrops below a minimum threshold level.
 5. A system for transmitting andreceiving a radio signal with a slotted frame structure, said systemcomprising: a first transceiver operable to transmit said radio signal;a second transceiver operable to receive said radio signal from aninitial antenna configuration selected from a plurality of possibleantenna configurations via antenna diversity; where said secondtransceiver can make a determination of reception quality for said radiosignal from at least one alternative antenna configuration selected fromsaid plurality of possible antenna configurations during the receptionof a slot containing data from said first transceiver in said initialantenna configuration; where said second transceiver can make aselection of an antenna configuration to receive said radio signal frombetween said initial antenna configuration and said at least onealternative antenna configuration; and where said determination ofreception quality is made at variable intervals, with said variableinterval being determined by the rate of fading experienced at saidsecond transceiver.
 6. A system for transmitting and receiving a radiosignal with a slotted frame structure, said system comprising: a firsttransceiver operable to transmit said radio signal; a second transceiveroperable to receive said radio signal from an initial antennaconfiguration selected from a plurality of possible antennaconfigurations via antenna diversity; where said second transceiver canmake a determination of reception quality for said radio signal from atleast one alternative antenna configuration selected from said pluralityof possible antenna configurations during the reception of a slotcontaining data from said first transceiver in said initial antennaconfiguration; where said second transceiver can make a selection of anantenna configuration to receive said radio signal from between saidinitial antenna configuration and said at least one alternative antennaconfiguration; where said second transceiver can transmit to said firsttransceiver using a different antenna configuration than the antennaconfiguration used for reception; where said second transceiverdetermines a transmission antenna configuration based upon informationreceived from said first transceiver during reception of said radiosignal, where said information indicates reception quality at said firsttransceiver; and where said second transceiver can transmit to a thirdtransceiver while receiving said radio signal from said firsttransceiver.
 7. The system of claim 6, where said second transceiverdetermines which of said first transceiver and said third transceiver totransmit to based upon information received from said third transceivervia said second transceiver.
 8. The system of claim 7, where said secondtransceiver determines which of said first transceiver and said thirdtransceiver to transmit to based upon a history of transmissionqualities achieved for each of said first and third transceivers.